1. Technical Field
The field addressed concerns high capacity optical fiber networks operative with wavelength division multiplexing. Systems contemplated: are based on span distances which exceed 100 kilometers; depend upon signal amplification rather than repeaters within spans, and use three or more multiplexed channels each operative at a minimum of 5.0 gbits per second.
2. Description of the Prior Art
The state of the art against which the present invention is considered is summarized in the excellent article, "Dispersion-shifted Fiber", Lightwave, pp. 25-29, Nov. 1992. As noted in that article, most advanced optical fiber systems now being installed and in the planning stages depend upon dispersion-shifted fiber (DS fiber). A number of developments have led to a preference for a carrier wavelength at 1.55 .mu.m. The loss minimum for the prevalent single-mode silica-based fiber is at this wavelength and the only practical fiber amplifier at this time--the erbium amplifier operates best at this wavelength. It has been known for some time that the linear dispersion null point--the radiation wavelength at which the chromatic dispersion changes sign and passes through zero--naturally falls at about 1.31 .mu.m for silica-based fiber. DS fiber--fiber in which the dispersion null point is shifted to 1.55 .mu.m--depends upon balancing the two major components of chromatic dispersion; material dispersion and waveguide dispersion. Waveguide dispersion is adjusted by tailoring the fiber's index-of-refraction profile.
Use of DS fiber is expected to contribute to multi-channel operation--to wavelength division multiplex (WDM). Here, multiple closely spaced carrier wavelengths define individual channels, each operating at high capacity--at 5.0 gbit/sec or higher. Installation intended for WDM either initially or for contemplated upgrading uses three or more channel operation, each operating sufficiently close to the zero dispersion point and each at the same capacity. Contemplated systems are generally based on four or eight WDM channels each operating at or upgradable to that capacity.
WDM systems use optical amplification rather than signal regeneration where possible. WDM becomes practical upon substitution of the optical amplifier for the usual repeater which depends upon electronic detection and optical regeneration. Use of the Er amplifier permits fiber spans of hundreds of kilometers between repeaters or terminals. A system in the planning stage uses optical amplifiers at 120 km spacing over a span length of 360 km.
The referenced article goes on to describe use of narrow spectral line widths available from the distributed feedback (DFB) laser for highest capacity long distance systems. The relatively inexpensive, readily available Fabry Perot laser is sufficient for usual initial operation. As reported in that article, systems being installed by Telefonos de Mexico; by MCI; and by AT&T are based on DS fiber.
A number of studies consider non-linear effects. (See, "Single-Channel Operation in Very Long Nonlinear Fibers With Optical Amplifiers at Zero Dispersion" by D. Marcuse, J. Lightwave Technology, vol. 9, No. 3, pp. 356-361, March 1991, and "Effect of Fiber Nonlinearity on Long-Distance Transmission" by D. Marcuse, A. R. Chraplyvy and R. W. Tkach, J. Lightwave Technology, vol. 9 No. 1, pp. 121-128, January 1991.) Non-linear effects studied include: Stimulated Brillouin Scattering; Self-Phase and Cross-Phase Modulation; Four-Photon Mixing (4 PM); and Stimulated Raman Scattering. It has been known for some time that correction of the linear dispersion problem is not the ultimate solution. At least in principle, still more sophisticated systems operating over greater lengths and at higher capacities would eventually require consideration of non-linear effects as well.